Discharge tube



May 19, 1970 HIDEO MIZUNO ETAL 3,513,349

DISCHARGE TUBE Filed Dec. 28, 1966 Frequency KC United States Patent US. Cl. 313-311 5 Claims ABSTRACT OF THE DISCLOSURE A coating for electrodes of discharge tubes including iron boride, a high-melting reducing metal such as zirconium, hafnium, niobium or tantalum, and electron emissive materials. The coating reduces R.F. emission in the 535-1605 kHz. range and is particularly adapted for lowpressure mercury-vapor discharge tubes.

This invention relates to discharge tubes and more particularly to improvements in low-pressure mercury-vapor discharge tubes such as fluorescent discharge lamps.

It is the primary object of the present invention to provide fluorescent lamps which radiate little electric noise (hereinafter referred to as the radio noise) in a radio frequency band, especially in a frequency range of 535 to 1605 kilocycles.

The means for accomplishing the foregoing objects and other objects, advantages and features of the present invention, which will be apparent to those skilled in the art, are set forth in the following description when read in conjunction with the accompanying drawing, in which the sole figure is a graphic illustration of the radio noise intensity resulting from the use of the inventive fluorescent lamps compared with that of radio noise resulting from the conventional fluorescent lamps.

Conventional oxide-coated cathodes employed heretofore as the electrodes of low-pressure mercury-vapor discharge tubes, such as fluorescent lamps, have been made with a double coil of tungsten or the so-called triple coil made by further winding a fine wire of tungsten around the above-mentioned tungsten double coil. A triple carbonate, consisting of barium carbonate, strontium carbonate and calcium carbonate, is coated on the double coil or the triple coil electrode. The electrodes are enclosed in a discharge tube, and during the step of exhausting the tube, this carbonate is thermally decomposed into oxides of barium, strontium and calcium and thus forms a socalled oxide-coated cathode.

The conventional oxide coating, made in the manner described above, has a high specific resistance and a poor thermal conductivity, resulting in the appearance of local high temperature cathode spots when the discharge tube is energized. These cathode spots act as emission centers. Due to thermal inertia, the temperature of a cathode spot hardly varies at the time of reignition and extinction of the arc, on each cycle of the alternating current, and remains high throughout the energized state of the discharge tube. As a result, the thermionic emission current I becomes greater than the discharge current I during the reignition and extinction of the arc on the cathode so that a negative field is formed in front of the cathode and causes cathode oscillation, which results in reignition noises and extinction noises. This cathode oscillation has been a major cause of severe radio interference.

Attempts have heretofore been made to improve the thermal conductivity of the cathode oxide coating to avoid the radio interference resulting from cathode oscillation in fluorescent lamps. Japanese patent publication No. 1581/ 1964, for example, discloses a method according to which the thickness of an oxide coating is reduced to a value of less than 30 microns thereby resulting in a substantial improvement of the thermal conductivity of such oxide coating and enlargement of the cathode spot to reduce the temperature thereof. This method, however, is undesirable because the absolute quantity of the oxide emitter is inevitably reduced and this leads to a short life for the discharge tubes. In another method disclosed in Japanese patent publication No. 8391/1965, temperature reduction of the cathode spot is attempted by widening the second pitch or second mandrel of the coil thereby avoiding an excessive temperature rise due to mutual radiation in the coil. Even with such a structure, the lamp delivers a radio noise on the order of 35 decibels and is still inadequate for operation as a noiseless fluorescent lamp.

In an effort to eliminate the above drawbacks inherent in conventional fluorescent lamps, the present inventors have discovered a new cathode material which can be used to reduce radio noise to less than 15 decibels. An improved fluorescent lamp can thus be obtained which is substantially perfectly noiseless in actual use and is free from blackening. The cathode material discovered by the present inventors consists essentially of iron boride powder which has a high melting point and a remarkably high thermal conductivity when compared with conventional ionic crystals, reducing metal powders such as zirconium, hafnium, niobium and tantalum, and emitters such as barium, strontium and calcium oxide. The above-described reducing metal powders, such as zirconium, hafnium, niobium and tantalum, effectively prevent the oxidation of iron boride during thermal decomposition of the carbonate during the step of exhausting the discharge tube.

According to an experiment made by the present inventors, a very excellent noise suppression effect could be obtained with a discharge tube provided with the inventive cathode material including iron boride (either FeB or Fe B) in an amount of 2 percent by weight, oxides of barium, strontium and calcium, and metallic zirconium, in an amount of 3 percent by weight of the oxides. When used in combination with a parallel connected condenser of 0.006 microfarad, this discharge tube performed in such an excellent fashion that a noise intensity of less than 15 decibels was attained, as shown by curve 2 in the figure, throughout a frequency range of from 535 kilocycles to 1605 kilocycles. The excellent performance of the inventive discharge tube is self evident through comparison with a corresponding performance of a conventional discharge tube, as shown by curve 1 in the figure.

As a matter of fact, it is not yet clear as to the reason why the addition of iron boride powder to the cathode material for an oxide-coated cathode is able to reduce the cathode oscillation and lower the radio interference to such a marked extent. But the fact that the cathode spot in the invention is widened, compared with that of a conventionally coated cathode, and that the temperature of the cathode spot is also lowered may be due to the reason that the thermal conductivity of iron boride is far higher than for oxides of barium, strontium and calcium. Considering also the excellent noise suppression eifect, there may be some sort of interaction between the iron boride and the electron emissive oxides.

The iron boride powder added to the oxides is highly resistant to heat, is chemically stable even at elevated temperatures, does not develop such phenomena as evaporation and sputtering, and does not cause any brittleness f the coil resulting from reaction with tungsten. Therefore, it is unlikely that undesirable conditions, such as extreme blackening and short lamp life, will be brought forth by inclusion of iron boride in the cathode oxide coating. Iron boride is normally represented by two chemical formulas, FeB and Fe B, which are both effective for use in the cathode material for the inventive electrode. The amount of boron required in the iron boride employed in the invention is not as strict as that defined by the chemical formulas. The boron content may lie in a wide range of from to 30 percent by weight.

In the inventive oxide-coated cathode, the addition of iron boride powder in an amount of more than 0.05 percent by weight exhibits a substantial effect for the prevention of radio interference, but an amount of the order of 0.5 to 3 percent is especially desirable to avoid possible content fluctuation in the manufacturing process and to obtain a cathode of good quality. Addition of iron boride in an amount of more than 3 percent by weight is still effective to attain the effect of noise suppression. However, the addition in an amount in excess of percent by weight is objectionable since addition of such a large amount results in reduction of the amount of oxides of barium, strontium, and calcium and in a shortened service life of the discharge tube. As described previously, it is essential that, in order to prevent blackening of lamps during their burning to thereby ensure a long service life, powders of at least one metal of a high-melting and reducing nature, selected from the group consisting of zirconium, hafnium, niobium and tantalum, should be added in an amount in the order of 1 to 8 percent to the oxide cathode material. Addition of these additives in a total amount of less than 1 percent is undesirable in view of the insufficient effect achieved while addition in a total amount in excess of 8 percent is also undesirable due to the fact that electron emission is lowered.

Several examples of composition of the inventive cathode coating material will be described hereunder:

EXAMPLE 1 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 28 Iron boride powder (FeB) 2 Metallic zirconium powder 3 EXAMPLE 2 Grams Barium carbonate a- 35 Strontium carbonate 35 Calcium carbonate 28 Iron boride powder (Fe B) 2 Metallic zirconium powder pup--. 3

EXAMPLE 3 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 28 Iron boride powder (FeB) 2 Metallic zirconium powder 2 Metallic hafnium powder 1 EXAMPLE 4 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 28 Iron boride powder (FeB) 2 Metallic zirconium powder 2 Metallic niobium powder 1 EXAMPLE 5 Grams Barium carbonate 35 Strontium carbonate 35 Calcium carbonate 28 Iron boride powder (FeB) 2 Metallic zirconium powder 2 Metallic tantalum powder 3 Each of the above coating compositions was suspended in a butyl acetate solution of introcellulose, and this suspension was coated on a double coil or a triple coil and subjected to thermal decomposition during the step of exhausting a discharge tube to thereby obtain a lowpressure mercury-vapor discharge tube having an oxidecoated cathode. In the above examples, barium carbonate, strontium carbonate and calcium carbonate were added in the form of a triple carbonate including these carbonates in such amounts as will give the above proportions.

Radio interferences of all the fluorescent lamps having the cathodes made in the above-described manner were less than 15 decibels in a radio frequency range of 535 to 1605 kilocycles, and the fluorescent lamps had a service life comparable to that of conventional fluorescent lamps. Thus, it will be appreciated that substantially perfectly noisless fluorescent lamps can be provided by the present invention.

The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The above-described embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore to be embraced therein.

What is claimed is:

1. A coating material for electrodes which substantially reduces radio interferences produces in electron discharge tubes consisting essentially of the mixture of iron boride powder in an amount of 0.05 to 10 percent by weight, at least one metal powder in an amount of 1 to 8 percent by weight, said metal being selected from the high-melting reducing metal group consisting of zirconium, hafnium, niobium and tantalum, and the balance being electron emissive material powders consisting of barium oxide, strontium oxide, calcium oxide or mixtures thereof.

2. A coating material for electrodes according to claim 1, in which said iron boride has the chemical formula, FeB.

3. A coating material for electrodes according to claim 11:, ill; which said iron boride has the chemical formula,

4. A coating material for electrodes according to claim 1, in which said iron boride is in the amount of 0.5 to 3 percent by weight.

5. An electrode for an electron discharge tube which has substantially no radiointerference consisting essentially of a wound coil of tungsten wire, a coating on said coil consisting of the mixture of iron boride powder in an amount of 0.05 to 10 percent by weight of said coating, at least one metal powder in an amount of 1 to 8 percent by weight of said coating, said metal being selected from the high-melting reducing metal group consisting of zirconium, hafnium, niobium and tantalum, and the bal- UNITED STATES PATENTS 2,737,607 3/1956 Lemmens et al. 313-346 3,312,856 4/1967 Lafferty et al. 313346 J. DAVID WELSH, Primary Examiner US. Cl. X.R. 

